Abstract: During somatic growth, increases in fish muscle mass occur both by hyperplasia and hypertrophy, and in some species the hypertrophic phase of growth leads to very large fiber diameters. Large fiber size leads is associated with low fiber surface area to volume (SA:V), which may (1) limit O2 flux across the sarcolemmal, (2) create potentially large intracellular diffusion distances between mitochondria, and (3) lead to diffusive constraints on aerobic metabolism. This study examined the effects of intracellular metabolite diffusion by investigating how the rate of post-contractile phosphocreatine (PCr) recovery in isolated epaxial white muscle from black sea bass (Centropristis striata) scales with body mass (and fiber size). Isolated muscle fibers from different sized fish (5.7 to 4159.4 g) were stimulated until PCr was depleted. The use of isolated fiber bundles in a high PO2 superfusion medium eliminated the confounding effects of fiber SA:V on oxygen flux across the membranes. Light microscopy and transmission electron microscopy techniques were employed to characterize fiber size and intracellular structure. White muscle fiber diameters increased significantly during growth, and became very large in adult fish (>250 µm). Mitochondrial density and cytochrome-c oxidase (COX) activity scaled negatively with increasing body mass, and had similar scaling exponents to each other. Sub-sarcolemmal mitochondrial volume per sarcolemmal membrane area significantly increased during fiber growth, indicating a shift in mitochondria towards areas of higher PO2 at the periphery of the fiber. Despite differences in fiber size, aerobic capacity, and intracellular diffusion distances between size classes, the post-contractile recovery rate of PCr was size independent. Further, a mathematical reaction-diffusion analysis indicated that the rate of PCr resynthesis was too slow to be limited by intracellular metabolite diffusion. These results suggest that in these white fibers, the rate of PCr recovery is limited by the low mitochondrial density. Additionally, the change in mitochondrial distribution with increasing fiber size suggests that low SA:V and limited O2 flux is a more important design constraint in large fibers of fish white muscle than is intracellular metabolite flux.